Organic Chemistry: The Alkenes  
An alkene is simply a C=C, that is a double bond between two carbon atoms. The position of the double bond will often determine what is the parent chain. If a double bond exists then the parent chain must include it. The double bond also uses up 2 bonds that normally hold hydrogen so the general formula for alkenes is CnH2n. The double bond also locks the molecule in a certain position so that attached groups can't rotate around the double bond. This means some special naming conventions.   Easy stuff first:
 Name            Formula
Ethene          CH2=CH2
Propene        CH2=CH-CH3
Butene          CH2=CH-CH2-CH3     or      CH3-CH=CH-CH3
There are two structural formulas for butene. This is because the double bond can come between the 1st and 2nd C or between the 2nd and 3rd carbon. We get around this by naming the position of the "ene". You'll also have noticed that the names have changed from "ane" to "ene" indicating the presence of a double bond.
CH2=CH-CH2-CH3     but-1-ene                    CH3-CH=CH-CH3      but-2-ene
These two are isomers. They have the same molecular formula, C4H8, but different structural formulas. We also have a new problem.

              cis-but-2-ene                                            trans-but-2-ene
The cis means (same side) and the CH3- methyl groups are both above the C=C bond and on the same side (top). Trans means transverse and the CH3- methyl groups are in a trans position to each other across the C=C bond.
Some more examples:
The longest chain of carbons is 7. Therefore this is a "hept" molecule. There is also a C=C double bond therefore it is a heptene. Counting from both ends the double bond starts at either 3 or 4. Choose the lowest number. The molecule is now hept-3-ene. The longest chain is also split around the C=C bond in a transverse fashion therefore it is also trans-hept-3-ene.
This accounts for everything about the parent chain. Since we choose the C=C number to be 3 we have now locked in place a numbering system for this molecule. That means there is a methyl group at carbon 4. The name of the molecule is therefore:   4-methyl-trans-hept-3-ene.
example 2:
Look for the longest continuous carbon chain. There are 9 carbons therefore it is a "nona" and there is a C=C at carbon 4 therefore it is non-4-ene. The sides of the chain of the parent are both on the same side. In this case, on top of the double bond therefore it is cis-non-4-ene. There is a methyl group at carbon 4 and an ethyl group at carbon 5. Therefore the full name is 5-ethyl-4-methyl-cis-non-4-ene.
         Go to Organic Alkene Series Worksheet